In telemeters of this type, the active laser material is illuminated by the flash of a discharge lamp, which causes fluorescent radiation to propagate along the same path as the laser pulse, the duration of this radiation being much longer than that of the laser pulse. Since such telemeters generally comprise optical emission and reception systems which are situated close to each other, the optical reception means can receive an interfering flash of the fluorescent radiation due to scattering of the fluorescent flash coming from the emission optical means. Although the power of the fluorescent radiation is much less than that of the laser pulse, this flash, by following a short optical path can reach the receiver of the telemeter at a power similar to that of a laser echo returned by a target and consequently it can cause a false measurement of its distance.
Preferred embodiments of the present invention mitigate this disadvantage.